Re: Low Voltage Tesla Coils

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "David Sharpe by way of Terry Fritz <twftesla-at-qwest-dot-net>" <sccr4us-at-erols-dot-com>

Terry
This is excellent and makes good sense!  Only issues I see
with this concept are:

1. Current limiting, but if you use a IGBT SS-Variac on
    front end in dynamic current limiting pulse by pulse, you've
    pretty much made the system blow-out proof.  "I" would
    still opt for a isolation transformer (read impedance
    "reactor" to limit current when the "OH SH..Ts" occur.
2.  Opto-couplers are limited to about 4kV hold off ("I"
     wouldn't feeld comfortable with only 0.3" spacing between
     1-2kV peak x 150-200A impulse.  Toshiba's IC opto
      is listed at Digikey (TLP250) and has ~60ns switching
     transistions and << 1microsecond delay.  It is good \
     enough to run a FET SSTC at 100kHz (although separate
     isolated power supplies are now needed).
3.  What is maximum surge current a 50A 1200V totem pole
      IGBT for motor control going to take before it goes POOF?
      Most are rated at 10 microsecond short circuit current
      ratings, and since you are decrement wave firing, peak
      currents up to perhaps 70% of this rating "may" be ok.
      I really get concerned about extreme silicon die thermal
      excursions at these obsince peak power levels.
4.  One to three turn primary, Vin =<1-1.5kV, Zo <=10 ohms.

Lets Roll!
Regards
Dave Sharpe, TCBOR
Chesterfield, VA. USA

Tesla list wrote:

> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>
> Hi Richard,
>
> Variacs have lots of head room so this does work, to a point (you'll "know"
> when you reach it ;-)).  The high voltage will saturate the core pretty
> fast.  However, who said we wanted a heavy expensive o'l variac either.
> With a bit of thought in the control, we can fire at any voltage we want
> just by timing when we trigger the IGBT during the primary cap charging
> cycle.  No need for a variac, but just another few IC's (we trust silicon,
> don't we? ;-))  Neat isn't it!! :-))  I suppose we could consider matrix
> converters too, but lets not get too "far out", yet ;-))
>
> But before you blow up a nice variac, consider this little circuit for
> getting close to 700 VDC at !terribly! high power!
>
> http://hot-streamer-dot-com/temp/Vx2.jpg
>
> BTW - These direct AC line circuits involve giant power levels!!!  Your
> grabbing the "power devil" by the tail...  Even pig system coilers will be
> surprised by the explosions these circuits can make!!!  Don't forget fuses,
> breakers, safety precaution, and getting far away if you dabble in such
> things!!!  The circuit above does not show breakers, soft start, drain,
> filtering, and a bunch of other very important "details"...
>
> A fine bit of advice for dealing with direct line currents:
>
> Imagine the side of your house where the breaker box is...
> Imagine a six foot hole getting blown through your house there...
> Imagine how far away you want to be when that happens..............
>
> Another note:
>
> I once asked a power engineer why the 12 Ga. line cord to his experiment
> seemed to be rather small?  He replied...  It's the fuse......  3 hours
> later, it proved to work...
>
> I suppose LVTC's will require a number of "new" safety notes...
>
> Cheers,
>
>         Terry
>
> At 08:38 PM 7/13/2002 -0400, you wrote:
> >> One may even be able to go down to ~300 or ~150 VDC
> >> (line voltage just rectified) and eliminate that transformer!!  An Off
> >Line
> >> Tesla Coil (OFTC - sorry again :-)))
> >
> >Terry, list,
> >
> >I've considered wiring a common variac as an auto transformer.  Probably
> >dangerous as hell, but that's what we do the best, eh?
> >
> >Put the full 220 vac across one HV leg and the wiper of say a 1256D and
> >take Vout from the other leg.  Would have to figure the lower limit of
> >wiper travel and put in a mechanical stop.  Wouldn't want to put 220 vac
> >and 150 amps across two windings.  Watts per variac wind could easily be
> >calculated and wiper travel limited to a safe range.  After all, we all
> >over drive variacs to 260-280 vac now.  What's a little more?
> >
> >Conceivably 500 to 800 vac could be available for your IGBT switched coil.
> >And, no heavy iron transformer.
> >
> >Comments?
> >
> >RWW
> >
> >
> >